A novel acetyl xylan esterase enabling complete deacetylation of substituted xylans.

BACKGROUND: Acetylated 4- O -(methyl)glucuronoxylan (GX) is the main hemicellulose in deciduous hardwood, and comprises a β-(1→4)-linked xylopyranosyl (Xyl p ) backbone substituted by both acetyl groups and α-(1→2)-linked 4- O -methylglucopyranosyluronic acid (MeGlc p A). Whereas enzymes that target singly acetylated Xyl p or doubly 2,3- O -acetyl-Xyl p have been well characterized, those targeting (2- O -MeGlc p A)3- O -acetyl-Xyl p structures in glucuronoxylan have remained elusive.

RESULTS: An unclassified carbohydrate esterase (FjoAcXE) was identified as a protein of unknown function from a polysaccharide utilization locus (PUL) otherwise comprising carbohydrate-active enzyme families known to target xylan. FjoAcXE was shown to efficiently release acetyl groups from internal (2- O -MeGlc p A)3- O -acetyl-Xyl p structures, an activity that has been sought after but lacking in known carbohydrate esterases. FjoAcXE action boosted the activity of α-glucuronidases from families GH67 and GH115 by five and nine times, respectively. Moreover, FjoAcXE activity was not only restricted to GX, but also deacetylated (3- O -Ara f )2- O -acetyl-Xyl p of feruloylated xylooligomers, confirming the broad substrate range of this new carbohydrate esterase.

CONCLUSION: This study reports the discovery and characterization of the novel carbohydrate esterase, FjoAcXE. In addition to cleaving singly acetylated Xyl p , and doubly 2,3- O -acetyl-Xyl p , FjoAcXE efficiently cleaves internal 3- O -acetyl-Xyl p linkages in (2- O -MeGlc p A)3- O -acetyl-Xyl p residues along with densely substituted and branched xylooligomers; activities that until now were missing from the arsenal of enzymes required for xylan conversion.